Recent years have seen the development of biometric authentication technology for authenticating a user of an apparatus or system based on a biometric image representing the user's biometric information such as a palm or finger vein pattern or a fingerprint or palm print. A biometric authentication device using such biometric authentication technology acquires, for example, as an input biometric image, a biometric image representing the biometric information of the user who wishes to use the biometric authentication device. Then, the biometric authentication device compares the user's input biometric information represented by the input biometric image with registered biometric information, i.e., biometric information represented in a biometric image for each registered user. If the result of the comparison indicates that the input biometric information matches the registered biometric information, the biometric authentication device authenticates the user as being a legitimate registered user. The biometric authentication device then permits the authenticated user to use the apparatus in which the biometric authentication device is incorporated or some other apparatus connected to the biometric authentication device.
In such biometric authentication technology, it is preferable that a characteristic structure of the biometric information is clearly captured on a biometric image, in order to compare data of a user in high accuracy. Therefore, a sensor which photographs the biometric information to generate a biometric image may include an illumination light source illuminating an object, such as a hand carrying the biometric information.
However, when a sensor photographs an object carrying the biometric information, the object being illuminated by environmental light such as sunlight, a biometric image may be unsuitable for a biometric authentication due to an influence of environmental light. For example, when the amount of light of environmental light entering into a sensor is large, a local luminance change on a biometric image resulting from a variation in the amount of environmental light is large, and therefore a luminance distribution may occur not depending on the biometric information on a biometric image. In particular, when a sensor directly receives strong environmental light, luminance values of pixels on a biometric image may be saturated, and a characteristic structure of a biometric image may not be captured on the biometric image.
In view of such a matter, there is a technique of adjusting luminance values of pixels of a biometric image by adjusting the amount of illumination light and exposure time at the time of photographing an object. For example, when a light emitting diode is used as an illumination light source, the amount of light per unit time is adjusted by adjusting period in which the light emitting diode is turned on according to a pulse-width modulation control of drive current. However, since the amount of emitting light of a light emitting diode changes according to an individual specificity, aging, or temperature, the light quantity may change even when current flowing through a light emitting diode is constant. Moreover, it is undesirable to excessively enlarge the amount of illumination light from a viewpoint of a security or power consumption. For example, a sensor may be connected to a computer through a Universal Serial Bus (USB) interface, and electric power supply may be made from the USB interface. In this case, since power consumption of an illumination light source is kept lower than an upper limit of the power consumption determined by the standard of USB, the amount of illumination light is also limited by the upper limit. Therefore, it is difficult to adjust the amount of illumination light according to the amount of light of environmental light.
In view of such a matter, techniques generating a biometric image has been developed by combining images photographed by a sensor according to environmental light of a circumference of the sensor (refer to Japanese Laid-open Patent Publication No. 10-222285, Japanese Laid-open Patent Publication No. 2001-357396 and Japanese Laid-open Patent Publication No. 2008-90412).
In an example of such known techniques, a target object image is extracted by calculating a difference component between a target object image when a target object is not illuminated by illumination light, and a target object image when the target object is illuminated by the illumination light.
In another example of the known techniques, a composed image in which biometric information is represented is generated by superimposing a predetermined number of accumulated images by an image compositing process unit so that an average luminance in a predetermined region is within a predetermined range of luminance, based on luminance information of an image obtained by photographing the biometric information by photographing means.
In still another example of the known techniques, predetermined exposure time is divided into a plurality of division exposure time. A plurality of illumination images are generated by subtracting luminance value of each pixel of an image obtained by illuminating and photographing an object carrying the biometric information for every division exposure time, from luminance value of each pixel of an image obtained by photographing the object without illumination. Then, an integrated image representing the biometric information is generated by integrating the plurality of illumination images.